DE 103 41 027 A1 which is incorporated by reference discloses a method for longitudinal control (ACC) which can be carried out in or with a two-circuit ABS/ESP motor vehicle brake control unit which is used on a widespread basis with a pump. In order to meter the hydraulic brake pressure, an analog-control, electrically actuable hydraulic isolating valve is used which, in contrast to control with an inlet valve which is actuated in analog fashion, permits pressure control over an entire brake circuit. The hydraulic valves which are used for pressure control are generally conventional, electromagnetically operated seat valves which are designed for switching. However, if these are actuated by means of a pulse-width-modulated (PWM) current in such a way that the valve tappet assumes a floating position, it is also possible to carry out analog control of the throughflow. However, to do this it is necessary to carry out the current control in a very precise and reproducible fashion. This is generally possible without relatively serious problems if the pressure to be applied can be applied via a pressure sensor in conjunction with a control loop. It is considerably more difficult to perform a corresponding analog control method without a pressure sensor in the pressure circuit which is to be controlled. In this case, use is frequently made of stored valve characteristic curves which, in conjunction with control methods which are known per se, permit pressure to be set in the wheel without additional wheel pressure sensors. The valve characteristic curve mentioned above therefore permits pressure to be set by means of the valve in accordance with a pressure request which is present at the controller as an input variable.
WO 2007/025951 (P 11192) which is incorporated by reference describes a method for determining an admission pressure prevailing between a master brake cylinder and an inlet valve of a wheel brake cylinder of a motor vehicle brake system, which method determines the admission pressure by taking into account the profile of a run-on voltage of a motor, operated in clocked fashion, of a pump which is used to feed back brake fluid from a low pressure accumulator into the master brake cylinder. In order to determine the admission pressure, electrical characteristic variables of the voltage profile are measured and respectively used to determine an admission pressure value. However, with this method it is still not yet possible to determine with sufficient precision the pressure which is brought about by the pump.
WO 2005/012056 A1 (P 10991) which is incorporated by reference discloses a method for determining the pressure of a fluid or the differential pressure which is present at an electromagnetically actuable control valve, in which method the pressure control valve is used to measure pressures. The pressure control valve comprises the customary components of an electrohydraulic pressure control valve such as an exciter coil, a valve tappet, valve domes, etc. The pressures are measured by means of an electronic control circuit which adjusts the position of the valve activation device or the magnetic force. The variable which is used for control purposes and is employed as an actual value represents the hydraulic force acting on the valve tappet. This force is measured by measuring the corresponding magnetic force which can be acquired from the voltage which is induced in the region of the exciter coil.